Estuarine microplastics under megacity pressure: methodological fragmentation, ecological exposure and management priorities in Can Gio Mangroves

Microplastic (MP) pollution in tropical mangroves is an emerging environmental concern, yet the mechanisms governing particle retention, remobilization, and ecological exposure remain poorly understood. This critical review synthesizes current evidence from the Can Gio Mangrove Biosphere Reserve (Vietnam) using a three-tiered evidence hierarchy to distinguish local observations, contextual evidence, and testable hypotheses. Existing studies reveal severe methodological fragmentation, including polymer distribution biases and the omission of nanoplastics and tire-wear particles, which prevents robust mass-balance budget closure. Consequently, Can Gio's role along the sink-source continuum remains unresolved. We propose the concept of a carrying-capacity paradox, whereby the same mangrove processes that enhance MP retention may also increase long-term ecological exposure by concentrating persistent particles within the ecosystem. This finding suggests that nature-based solutions alone cannot substitute for upstream source reduction. Ecotoxicological evidence is further constrained by limited exposure realism, as most studies rely on laboratory bioassays conducted under conditions that poorly represent the natural environment. To address these limitations, we propose a phased research roadmap integrating methodological standardization, mass-balance assessment, and native-species risk calibration. Collectively, these findings provide an evidence-based framework for improving MP monitoring, ecological risk assessment, and management of tropical mangrove ecosystems.